New theory rewrites opening moments of Chernobyl disaster

A picture of the HDR technology of the Sarcophagus in the Chernobyl Zone. Credit: Piotr Andryszczak

A brand-new theory of the opening moments during the Chernobyl disaster, the most severe nuclear accident in history, based on additional analysis is presented for the first time in the journal Nuclear Technology, an official journal of the American Nuclear Society.

The new theory suggests the first of the two explosions reported by eyewitnesses was a nuclear and not a steam explosion, as is currently widely thought and is presented by researchers from the Swedish Defence Research Agency, Swedish Meteorological and Hydrological Institute, and Stockholm University.

They hypothesize that the first explosive event was a jet of debris ejected to very high altitudes by a series of nuclear explosions within the reactor. This was followed, within three seconds, by a steam explosion which ruptured the reactor and sent further debris into the atmosphere at lower altitudes.

The theory is based on new analysis of xenon isotopes detected by scientists from the V.G. Khlopin Radium Institute in the Leningrad, four days after the accident, at Cherepovets, a city north of Moscow far from the major track of Chernobyl debris. These isotopes were the product of recent nuclear fission, suggesting they could be the result of a recent nuclear explosion. In contrast, the main Chernobyl debris which tracked northwest to Scandinavia contained equilibrium xenon isotopes from the reactor's core.

By assessing the weather conditions across the region at the time, the authors also established that the fresh xenon isotopes at Cherepovets were the result of debris injected into far higher altitudes than the debris from the reactor rupture which drifted towards Scandinavia.

Observations of the destroyed reactor tank indicated that the first explosion caused temperatures high enough to melt a two-meter thick bottom plate in part of the core. Such damage is consistent with a nuclear explosion. In the rest of the core, the bottom plate was relatively intact, though it had dropped by nearly four meters. This suggests a steam explosion which did not create temperatures high enough to melt the plate but generated sufficient pressure to push it down.

Lead author and retired nuclear physicist from the Swedish Defence Research Agency, Lars-Erik De Geer commented, "We believe that thermal neutron mediated nuclear explosions at the bottom of a number of fuel channels in the reactor caused a jet of debris to shoot upwards through the refuelling tubes. This jet then rammed the tubes' 350kg plugs, continued through the roof and travelled into the atmosphere to altitudes of 2.5-3km where the weather conditions provided a route to Cherepovets. The steam explosion which ruptured the reactor vessel occurred some 2.7 seconds later."

Seismic measurements and an eye-witness report of a blue flash above the reactor a few seconds after the first explosion also support the new hypothesis of a nuclear explosion followed by a steam explosion. This new analysis brings insight into the disaster, and may potentially prove useful in preventing future similar incidents from occurring.

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This isn't exactly news.If I remember correctly their control rods had both neutron absorbers and moderators at different positions on the same rod! It was an accident waiting to happen.(Like a car with the gas and brake pedals taped together.)

Then they were trying a worst case scenario experiment and the heat distorted the core and jammed the rods in go faster mode.

It is, and a bit incredible because you need to squeeze the fuel together pretty tight and pretty fast in order to initiate a nuclear explosion. Otherwise the heat and the radiation pressure forces the fuel apart and self-moderates the reaction down. Of course it gets really hot in there and the pressure rises because things melt and turn into gasses, but it's not a micro-sized nuclear bomb.

In nuclear weapons, the radioactive fuel is forced together with high explosives. If it isn't squeezed in fast enough, hard enough, in the correct shape, you get a nuclear fizzle instead of a nuclear explosion, and it isn't easy to make it go - and that's with highly enriched fuel, much stronger than in a nuclear reactor.

So it's hard to believe that an actual nuclear explosion, like in nuclear weapons, happened inside Chernobyl.

Their explosion would be a 'fizzle' a protracted criticality event.3MW gone rogue to 30MW is a big fizzle though.You're right about the need for an unusual localised structure being needed for a nuclear explosion. Usually they just melt down, generate steam and hydrogen and pop the containment.They were trying a fuel arrangement outside of the reactors design and it went prompt critical.

Point being that the criticality event was slow enough to be registered at all by the plant instruments. By the time things really started breaking down and the explosions happened, the reaction must have been already winding down as the heat and radiation was pushing the fuel apart.

If it had been a nuclear explosion, the power excursion would have continued straight on to blowing the thing up, instantly. No runaway reactor and people scrambling around - just "whoops" - "bang".

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